The present invention generally relates to gas generators such as used to inflate air bags in an automobile occupant protection system, and more particularly to a gas generator having an improved end cap assembly.
Conventional gas generators are typically designed with an end closure integral to a housing on one end, and a second end closure at an opposite end. The outer housing of the generator usually encapsulates a filter, an ignition means, and a propellant bed. The second end closure is normally fixed to the housing once propellant grains have been inserted into the propellant chamber, thereby sealing the completed generator assembly.
One disadvantage is that the generator assembly often tends to rattle due to spatial gaps inherent within this design. As a result, the noise within the vehicle is increased. During operation of the vehicle, vibrations within the vehicle cause corresponding vibrations within the gas generator. Friction caused by continuous vibration of the generator assembly may lead to deterioration and failure of the o-ring(s) or other sealant means. Loss of the generator seal reduces the pressure of the propellant chamber upon combustion, and consequently, inhibits the ignition and combustion of the propellant grains upon collision. Loss of the hermetic seal may also result in a reduction of the propellant burn rate due to absorption of moisture within the propellant bed. Poor ignitability, a reduced burn rate, and poor sustained combustion lead to increased performance variability and a greater likelihood of inflator inoperability.
Performance variability may also be increased by fragmented propellant grains resulting from generator assembly vibrations. The propellant grains are shaped and sized to burn at a predetermined rate. Fragmentation of the propellant pellets skews the designed burn rate and interferes with continuous combustion within the propellant bed which in turn may adversely affect inflator operability.
The problem of vibration has been previously addressed by incorporating an integral end closure comprising a polymer seal and spring subassembly. The spring applies a constant pressure axially parallel to the propellant bed thereby dampening the vibration within the bed. The polymer based seal ensures that the combustion gases flow through their predetermined path rather than through gaps that may have remained in the crimped end closure. Without the seal, welding, as opposed to crimping, is necessary to close the gaps and prevent "blowby". Consequently, manufacturing costs are increased.
Although the end closure comprising the polymer seal and spring assembly represents an improvement over conventionally designed generators, it too has its disadvantages. For example, certain polymer or silicone based seals react with the heat of combustion resulting in the release of fluorocarbons and in the production of other undesirable gases. Furthermore, the generator assembly vibrations are not significantly dampened by the spring assembly and as such, potential o-ring failure as well as increased noise are still cause for concern.
Therefore, a need still exists for a gas generator that is resistant to vibration, is environmentally sound, and has reduced performance variability.